WASHINGTON — As he pleaded guilty in a Manhattan courthouse on Tuesday to violating federal campaign finance laws, Michael D. Cohen, President Trump’s longtime fixer, put his future in the hands of the American legal system.
But the fate of Mr. Trump, the man who Mr. Cohen said directed him to break the law by making payments to a pornographic film actress and a former Playboy model, rests, in all likelihood, in the political arena and in the halls of Congress.
At least for now, the Republican Party continues to stand by the president. But with only weeks until the midterm elections, the question will soon be put before voters, who will decide whether to hand Congress — and the power of investigation, subpoenas and, possibly, impeachment — to the Democrats.
After Mr. Cohen’s pleas and a guilty verdict, minutes later, in the bank and tax fraud case against Mr. Trump’s former campaign chairman, Paul Manafort, Democrats wasted no time in demanding a congressional investigation into the Cohen affair and warned of an increasingly dangerous threat to the rule of law.
“This is getting deeper and deeper, and it’s going to get more and more serious,” said Representative Jerrold Nadler of New York, the top Democrat on the House Judiciary Committee. Asked about the potential for an impeachment inquiry, Mr. Nadler said he wanted to see more evidence.
“We need to see what Mueller comes up with,” he said, referring to Robert S. Mueller III, the special counsel investigating the Trump campaign’s ties to Russia and whether Mr. Trump obstructed justice. “We may get there.”
Senator Ron Wyden, Democrat of Oregon, broached the charged phrase “high crimes and misdemeanors,” the Constitution’s threshold for impeachment.
But lawmakers in Mr. Trump’s party, who have repeatedly brushed away concerns about Mr. Trump’s own legal exposure, seemed unperturbed. With the House away from Washington for the month and senators sick of the drama emanating from Mr. Trump’s orbit, few openly rose to Mr. Trump’s defense. Those who did said the threat was elsewhere.
“Campaign finance violations — I don’t know what will come from that, but the thing that will hurt the president the most is if, in fact, his campaign did coordinate with a foreign government like Russia. Anything short of that is probably going to fall into partisan camps,” Senator Lindsey Graham, Republican of South Carolina, told reporters. As a House member, Mr. Graham helped prosecute the impeachment of President Bill Clinton two decades ago.
Senator John Cornyn of Texas, the No. 2 Senate Republican, repeatedly stepped around questions from reporters about the implications of Mr. Cohen’s pleas. He observed instead that independent counsel investigations often tended veer away from their origins.
“I have no idea about what the facts are surrounding his guilty plea other than the fact that none of it has anything to do with the Russia investigation,” Mr. Cornyn said. “I would make the same observation with regard to Mr. Manafort.”
Aides to Speaker Paul D. Ryan, Republican of Wisconsin, said Mr. Ryan was “aware of Mr. Cohen’s guilty plea to these serious charges” but would wait for more information to comment further. Senator Mitch McConnell, Republican of Kentucky and the majority leader, said nothing publicly.
In a sign of how cemented both the opposition to and support for Mr. Trump is a year and a half into his presidency, few Republicans believed the double-digit felony count would drastically reshape the political climate. Party strategists largely dismissed Mr. Manafort’s conviction, viewing it as not directly related to Mr. Trump, and said voters had already concluded that the president agreed to pay off the actress, Stephanie Clifford, better known as Stormy Daniels.
“I’d be surprised if it causes anything more than a ripple in the campaign,” said Steven Law, who oversees the Senate Leadership Fund, the “super PAC” aligned with Mr. McConnell.
Part of what the president has going for him is that expectations about his conduct were already low before Tuesday’s split-screen drama, with Democrats believing he is an amoral demagogue and many Republicans seeing him as an unsavory character who has the right policies and, just as important, the right enemies.
“In a normal world, it’s not good,” said Chris Wilson, a Republican pollster. “In today’s political environment, it probably just creates further polarization and political tribalism.”
This is not to say that the president’s approval ratings will not dip slightly or that preferences for a Democratic-controlled Congress will not rise on the news: Many Republicans believe they are already likely to lose control of the House and that such Trump-linked wrongdoing will only further enhance Democrats’ chances for a takeover.
But given the health of the economy and relative stability abroad, Mr. Trump’s approval ratings are already significantly lower than what virtually any other president would be enjoying at this moment of his administration. And if there is one predictable element of this otherwise unpredictable presidency, it is that some new story will detonate in the days or weeks ahead, pushing the last eruption off the home page and television chyron.
That fact was both a tonic to Republicans on Tuesday and a reason for concern. While suggesting that the Manafort and Cohen felonies were merely this week’s version of the Omarosa tapes — the latest ephemeral drama to grip Washington but barely faze voters — Republican officials were also apprehensive about what more Mr. Cohen may reveal on his way to a likely prison sentence.
And even more worrisome to Republicans is what damage Mr. Trump may do to himself. If the president is seen as thwarting Mr. Mueller’s investigation, either by terminating the special counsel or pardoning allies who are implicated, it would create a far more serious upheaval and force Republican lawmakers into a confrontation they have long avoided.
“Of course you can imagine the president doing things that would be counterproductive,” said Senator Bill Cassidy, Republican of Louisiana, before adding with evident hope in his voice, “But you also could imagine the president saying something but never acting on it, just venting.”
Privately, Democrats speculated that the guilty pleas and decisions stacking up around Mr. Trump would fuel a message that Republicans controlling all levers of Washington were woefully corrupt.
Their case got another boost only hours later when a federal grand jury indicted Representative Duncan Hunter of California on charges that he spent tens of thousands of dollars in campaign funds on personal expenses. Mr. Hunter was the second Republican congressman to be federally charged in just three weeks and, taken together with the Cohen and Manafort felonies, some Republicans thought the misdeeds carried a stench reminiscent of the 2006 elections, when Democrats last reclaimed control of Congress.
“Today’s events are a bombshell, and if nothing shakes them loose from their purposeful inertia, the electorate will do so,” Senator Richard Blumenthal of Connecticut, one of Mr. Trump’s most outspoken Democratic critics, said of the Republicans.